The present invention relates to control systems for vehicular spreaders; and more particularly, it relates to a control system for a spreader of the type used in dispensing material from a moving vehicle in which the amount of material being spread is coordinated with the speed of the vehicle. That is to say, it is desirable that the amount of material spread over a given area be controlled according to the speed of the vehicle to yield a constant coverage per unit area, while allowing the vehicle speed to change over a considerable range.
Systems of this type are used to spread sand or other abrasive materials or ice control chemicals on highways, airport runways and the like, or to distribute agricultural chemicals such as fertilizers or agricultural limestone in dry or liquid form.
In such systems, a large hopper is mounted on a truck, and equipped with a conveyor or other delivery means which carries material from the hopper and delivers it to a discharge aperture, and feeds the material to one or more spinners at the rear of the vehicle. Alternatively, for the spreading of liquid material, the liquid may be stored in a tank and a variable speed pump used to supply the material under pressure to a plurality of nozzles mounted on the rear of the vehicle. The ground speed of the vehicle is sensed and a signal, either electrical, mechanical, or hydraulic, which is representative of the ground speed is generated and used to actuate a control valve associated with a variable speed hydraulic motor which drives the delivery means. As the vehicle speeds up, the speed of the conveyor motor is increased, and vice versa. Early systems of this type were "open loop" systems; and by this it is meant that the control valve was actuated directly by the ground speed signal. Other systems also sense the speed of the conveyor, and use an error signal representative of the difference between the speed of the vehicle and the speed of the conveyor to actuate the control valve. These are referred to as "closed loop" or feedback systems. Systems of this type may be further characterized as hydro-electronic control systems or hydro-mechanical control systems. A hydro-electronic control systems is disclosed in the Wilder, et al U.S. Pat. No. 3,344,993, issued Oct. 3, 1967.
One embodiment disclosed in this patent includes an electronic control unit which compares a first electrical analog signal representative of vehicular ground speed and a second electrical analog signal representative of the speed of the conveyor motor. An error signal is used to modulate a control valve to adjust the speed of the conveyor motor to bring two compared analog signals into equilibrium. These systems have the disadvantages of high initial cost, the difficulty in accurately comparing analog signals, a tendency to "hunt" which results in irregular material distribution, and a low speed threshold which is unacceptably high. For example, a low speed threshold of about three miles per hour is not suitable for some highway and agricultural purposes, and it must be considered that these systems have a broad range of application, particularly in the variations of useful speed of the vehicle. Further, commercially available electronic feedback systems which are all electronic are subject to high maintenance costs caused primarily by the hostile environment in which the systems normally operate. That is to say, in the case of an agricultural spreader, the corrosive effects of a fertilizer on the electronic transducer used to sense ground speed may cause an unacceptably high rate of failure. Salt and ice control chemicals have a similar corrosive effect.
A hydro-mechanical feedback control spreader system is disclosed in the co-owned Rawson U.S. Pat. No. 3,441,039, issued Apr. 29, 1969. In this system, ground speed is sensed by means of a friction drive wheel which is in contact with the ground or with a wheel of the vehicle, for example. Conveyor speed is sensed by direct drive from the conveyor motor or its gear train. The two rotational signals are compared in a mechanical comparator such as a rotating screw and nut, as disclosed in the patent, in which equal speeds of rotation will maintain the nut in a fixed axial position on the screw; whereas if one rotational speed varies from the other, the nut will migrate along the axis of the screw, and this displacement is used to control the speed of the conveyor motor accordingly. In systems of this type, the main disadvantages are the low torque capability of speedometer drives (which provides the energy for driving either the nut or the screws), the complexity of transferring a ground speed rotational signal without error to the mechanical comparator, and the difficulties of installation. Such systems have been found capable of operating in hostile environments with relatively low maintenance requirements, however.
A more recent improvement in a closed loop hydro-mechanical spreader control system is disclosed in the co-owned U.S. Pat. No. 3,904,129, issued Sept. 9, 1975. This system uses a mechanical comparator in the form of a differential control valve using a rotary spool for flow control to the hydraulic motor.
The present invention is an improvement in the hydro-mechanical closed loop spreader control system of the type just mentioned. Specifically, an electronic link is incorporated for generating a train of pulses from a low-torque transducer wherein the repetition rate of the pulses is representative of vehicular ground speed. The ground speed transducer may be an optical shaft encoder employing a rotary disc driven by the speedometer shaft and interrupting a photo coupler to generate the pulses. As the speed of the vehicle increases, the repetition rate of the pulses generated by the transducer also increases.
The output pulses from the transducer, after amplification, energize a stepper motor which, in turn, drives an input to the mechanical comparator. The other input to the mechanical comparator is derived from the motor which drives the conveyor. If it is desired to proportion the pulses from the transducer, the output of the amplifier may be fed to a counter circuit for dividing the repetition rate of the pulses. Thus, the transducer or rotary shaft encoder, amplifier, counter circuit, if any, and stepper motor comprise an electronic digital link in the feedback control system. The stepper motor achieves synchronization between the digital ground speed signal and the input to the mechanical comparator over the full range of speeds encountered, and particularly, from a speed slightly above zero miles per hour to any practicable maximum speed. In brief, the use of a digital electronic link in the feedback control system extends the response range of the system and increases its accuracy, particularly at lower speeds.
The capability of proportioning output pulses through the use of a counter will accommodate the present system for all types of material to be spread. That is, it will permit the conveyor to be run at a maximum output for a low ground speed for high volume distribution, such as at 71/2 miles per hour as is sometimes the case in spreading limestone at rates of several tons per acre or at lower volume distributions at intermediate speeds, such as in the spreading of fertilizer at rates of a few hundred to a thousand pounds per acre and at speeds up to thirty miles per hour. It will also be useful for low volume distribution at highway speeds, for example, the spreading of a few hundred pounds of sand or salt per mile for ice control at speeds up to 45 miles per hour. It is normally the case that the conveyor is run at maximum delivery speed when the vehicle is at the maximum speed for its range, and the use of a counter in the digital electronic link facilitates achieving maximum conveyor speed at the maximum vehicle speed.
The use of an electronic digital link permits the use of electrical wires with the corresponding ease of installation from the operator's station to the mechanical comparator without requiring substantial modification of the structure of the vehicle.
Modifications of the system are disclosed including an override circuit in the digital electronic link, including a selector switch actuated by the operator which will generate a train of digital signals causing the stepper motor to run at maximum speed irrespective of the actual ground speed of the vehicle. This is useful in the spreading of ice control materials where heavy distribution is desired, such as at intersections or over bridges. The override circuit may include an oscillator for generating a train of pulses having a repetition rate corresponding to the maximum speed of the stepper motor. It may also be provided with a time cut-off so that the heavy material distribution will occur only for a specific time after the override switch is actuated. The cut-off time may be adjustable. In another embodiment, the override cut-off is controlled by vehicle displacement such that once the override is actuated, it will continue until the vehicle has travelled a predetermined distance. In this case, the cut-off may also be adjustable.
Two features of digital transducers, such as the one disclosed herein, are that the rate of digital pulses generated is independent of the direction of travel of the vehicle so that the same system may be used for spreading in the reverse direction, and the amplitude of the measuring signal is independent of the speed of the vehicle. The first is useful in applications where it is desired to spread the material in limited access areas; and the second overcomes a problem in the prior art analog system mentioned.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of preferred embodiments accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.